This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remote controlled electro-mechanical impedance tuners. Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” (see ref. 1). Load pull is a measurement technique employing microwave tuners and other microwave test equipment as shown in FIG. 1. The microwave tuners 2, 4 are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor, 3) is tested, see ref. 1; the signal is provided by a signal source 1 and the outcoming power is measured by a power meter 5; the whole is controlled by a PC 6, which comprises interfaces to communicate with the instruments and the tuners, using digital cables 7, 8 and 9. This document refers hence to “impedance tuners”, see ref. 2, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits.
Two-carriage impedance tuners comprise, in general, a transmission line 30, FIG. 3, and a number of conductive tuning elements (probes, 31, 32) attached 22 on adjustable vertical axes 33, 34, which, when approaching 26 the center conductor 23 of the slabline 24 (FIG. 2) and moved 25 along the axis of the slabline, create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is 50Ω.
When cubical metallic probes (slugs) 21 with a concave bottom approach the center conductor 23, they capture the electric field, which is concentrated in the area between the center conductor and the ground planes of the slabline 24, FIG. 2. This field capturing allows creating high and controllable reflection factors. The disadvantage of this technique is the requirement of high precision and resolution cumbersome vertical 26 probe movement mechanisms 33, 34. Because most of the field capturing effect occurs when the probe is very close to the center conductor a high-resolution vertical control mechanism is needed. This, on the other hand, not only slows down the tuning procedure, since, when the probe is away from the center conductor, the tuning effect is much less prominent, but the vertical moving speed is the same (see FIG. 15 in ref. 7), but also requires enhanced positioning accuracy due to high tuning sensitivity in the high reflection area, when the slug is very close to the center conductor.
A further disadvantage of traditional multi-carriage tuners is their linear length. It must be at least one half of a wavelength (λ/2) per carriage, i.e. in the case of a two-carriage (harmonic) tuner this would be one full wavelength. At 400 MHz lowest frequency the total length would be 75 cm. The present invention discloses a structure, whereby the slabline is bottom-less and the disc probes are inserted from opposite sides, sharing the same length of line and allowing a shrinking in length by a factor of 2 (see ref. 3). However, a structure as in ref. 3, still requires cumbersome vertical axes cancelling in tuner width what is gained in length. Therefore the present solution employs disc tuning probes, see ref. 7, which offer a number of advantages, in addition to the elimination of the vertical axes 33, 34.